In an area there can be one or more telecommunications networks with different technologies operated by one or more telecom operators. Such an environment increases the complexity of a network selection by the user equipment (UE), but it is also inefficient from operators' point of view due to high investment costs of a network or part of it. In order to improve the efficiency the operators may have agreed on sharing at least part of their networks, e.g. in a situation when a network is owned by some companies in a joint ownership basis. As a result, operators may establish an own network for certain areas, but co-utilize a network in some other areas in order to provide extensive communication services to the subscribers.
There exist different models for the network sharing. Mainly, the models relate to the extent of sharing the network and/or part of it. In principle, it is possible to share any part of the network, but basically one can talk about sharing a core network, radio network or both of them. The sharing shall also be understood as covering a share of at least one network element and/or radio resources. More specifically, a network sharing architecture shall, according to at least 3GPP Specification TS 23.251 V11.0.0 (2011-09), allow different core network operators to connect to a shared radio access network. The operators do not only share the radio network elements, but they may also share the radio resources, i.e. sharing spectrum.
In order to utilize shared networks the UE must be capable of it. In practice this means that the UE must be able to receive and utilize the additional broadcast system information concerning available core network operators in the shared network. The term ‘core network operator’ shall be understood as an operator that provides services to subscribers as one of multiple serving operators that share at least a radio access network. The additional broadcast system information comprises identities of the network operators involved in the network sharing. The network operators in the shared environment are identified with public land mobile network identities (PLMN-id), which consists of a Mobile Country Code (MCC) and a two to three digit Mobile Network Code (MNC). In shared UTRAN, the network operators PLMN identities are broadcasted in a Multiple PLMN list information element. When the term multiple PLMN or multiple PLMN-id is used in this document it refers to the PLMN identities which are broadcasted in a Multiple PLMN list information element.
Based on the capability of utilizing the additional broadcast system information the user equipments can be categorized into two groups. First group are such UEs, which support network sharing in the sense that they are able to select a core network (CN) operator as the serving operator within a shared network on the basis of the additional broadcast system information. Such UEs are called supporting UEs in this context. Correspondingly, second group of UEs i.e. non-supporting UEs do not support network sharing in the sense that they ignore the additional broadcast system information that is specific for network sharing, i.e. the PLMN identities of the network operators. In that case, the UEs read common PLMN, which is the PLMN identity indicated to the UE as the serving operator.
The importance of being aware of the capability of a UE by a network becomes obvious in a mobility situation. Namely, if a UE is moving from a non-shared network to a shared network, it would be advantageous to know in advance if the UE supports network sharing or not. With this information the network is able to optimize radio resource management strategy accordingly. For example, the network may select to handover a UE to a RAT or frequency band that is not in a shared network configuration if the UE does not support network sharing to at least a certain RAT. The source network may also perform prioritization at e.g. Circuit Switched (CS) fallback or redirect or cell change order or cell reselection. The advantage in avoiding sending a non-supporting UE to a shared network is that non-supporting UEs are subject for CS/PS (Packet Switched) coordination, and in this process the subscriber may be lost to another operator that is part of the shared network. Another advantage in knowing in advance if the UE supports network sharing or not is that it is then possible to arrange the source network to inform the target network into which PLMN identity the UE in question shall be registered to. If the capability information of the UE is not known, the target network may provide both multiple PLMN-ids and the identity of the common PLMN to the UE, but does not know if the UE will use one of the multiple PLMNs or the common PLMN, at least as long as the UE has an established NAS (Non Access Stratum) signaling connection in the target network. In other words, since the target network does not know which PLMN the UE will use, it is not capable of determining if the UE ought to be registered to the Common PLMN or to one of the PLMN identities in the Multiple PLMN List. If the target network registers all UEs in a multiple PLMN-id then non-supporting UEs will be registered to wrong PLMN, since they are supposed to be registered in the common PLMN. If the target network registers all UEs in the common PLMN then supporting UEs will be registered to wrong PLMN, since they are supposed to be registered in a multiple PLMN-id. When the NAS signaling connection ends, the supporting UE indicates a PLMN identity, which thus indicates the capability of the UE. If the target network handles all UEs as non-supporting UEs then supporting UE may be subject of rerouting (CS/PS coordination) to another operator than intended.
As said, when the capability of the UE is known by the non-shared network in the handover situation, the PLMN to be used for registration in shared cell can be optimally selected by signaling between the non-shared and shared networks. More precisely, the non-shared network may use the information at handover signaling towards the shared network. The information may be used in taking decisions in source network, for example, with respect to the handover (or other mobility procedures) of non-supporting UEs to another network than the shared network if that is preferred and possible.